The development of highly absorbent articles is the subject of substantial commercial interest. A highly desired characteristic for such products is dryness. This can be achieved by removing the fluid from the proximity of the wearer's skin. Dry diapers, for example, can be more comfortable to wear and less prone to cause skin problems such as rash (diaper dermatitis). The same advantages also accrue for feminine hygiene pads and tampons, adult incontinence briefs, pads for managing perspiration, bandages for wounds, and the like, which generally comprise the class of absorbent articles as worn by humans or animals. Other absorbent articles include items used in construction for leakage prevention, in surgery for blood management, and in horticulture and agriculture for controlled water delivery to plants.
The ability to provide drier absorbent articles such as diapers is contingent on having absorbent cores or structures that can acquire, distribute, and store discharged body fluids such as urine. Such cores can comprise two or more components optimized for the specific function. For example, the core may be a composite of layers which are designed to acquire the fluid quickly to manage gushes or sudden insults, distribute the fluid to regions distant from the point of original insult, and store the fluid by locking it in irreversibly at that distant point.
The art is replete with examples of storage materials, including hydrogel-forming absorbent polymer (hereafter referred to as "HFAPs". These materials are also referred to in the art as "hydrocolloids", "supersorbers" or superabsorbent polymers. The use of such materials is summarized in "Water-Absorbent Polymers: A Patent Survey", Po, R. J.M.S.--Rev. Macromol. Chem. Phys. 1994, C34(4), 607-662. Such storage materials are usually blended with a fibrous web in varying proportions for use in absorbent cores. A representative example of such HFAPs are lightly crosslinked polyacrylates. Like many of the other absorbent polymers, these lightly crosslinked polyacrylates comprise a multiplicity of anionic (charged) carboxy groups attached to the polymer backbone. It is these charged carboxy groups that enable the polymer to absorb aqueous body fluids as the result of osmotic forces.
Fluid storage can also be based on capillary forces. Capillary forces are notable in various everyday phenomena, as exemplified by a paper towel soaking up spilled liquids. The art is replete with examples of capillary absorbents formed from fibrous assemblies. Another type of capillary absorbent is sphagnum moss, as described in EP 779,066 (Chauvette et al.) published Jun. 18, 1997. Capillary absorbents can offer superior performance in terms of the rate of fluid acquisition and wicking, i.e., the ability to move aqueous fluid away from the point of initial contact. Indeed, the dual-layer core absorbent structures noted above use the fibrous matrix as the primary capillary transport vehicle to move the initially acquired aqueous body fluid throughout the absorbent core so that it can be absorbed and retained by the HFAP positioned in layers or zones of the core. However, such constructions when used as storage elements can fail under pressure leading to expressed or free fluid.
The use of absorbent foams in absorbent articles such as diapers has been found highly desirable. If made appropriately, open-celled hydrophilic polymeric foams can provide useful capillary fluid acquisition, transport and/or storage required for use in high performance absorbent cores. Absorbent articles containing such foams can provide desirable wet integrity, sustained body fit, minimize changes in shape during use (e.g., swelling, bunching), and skin dryness. Dryness is achieved by use of layers of foam having differential capillary pressures so that most of the fluid ends up in the layer most distant from the wearer's skin. Examples of these designs are described in U.S. Pat. No. 5,563,129 (infra).
The most useful absorbent foams for absorbent products such as diapers have been made from High Internal Phase Emulsions (hereafter referred to as "HIPEs"). See, for example, U.S. Pat. No. 5,260,345 (DesMarais et al), issued Nov. 9, 1993, U.S. Pat. No. 5,268,224 (DesMarais et al), issued Dec. 7, 1993, U.S. Pat. No. 5,387,207 (Dyer et al.) issued Feb. 7, 1995, U.S. Pat. No. 5,652,194 (Dyer et al.) issued Jul. 29, 1997, U.S. Pat. No. 5,563,179 (Stone et al.) issued Oct. 8, 1996, and U.S. Pat. No. 5,563,129 (supra) and U.S. Pat. No. 5,560,222 (DesMarais et al.) issued Jul. 22, 1997, the disclosure of each is included herein by reference. These absorbent HIPE foams provide generally desirable fluid handling properties, including: (a) relatively good wicking and fluid distribution characteristics to transport the imbibed urine or other body fluid away from the initial impingement zone and into the unused balance of the foam structure to allow for subsequent gushes of fluid to be accommodated; and (b) a relatively high storage capacity with a relatively high fluid capacity under load, i.e., under compressive forces. These HIPE-derived absorbent foams are also sufficiently flexible and soft so as to provide a high degree of comfort to the wearer of the absorbent article, and can be made relatively thin until subsequently wetted by the absorbed body fluid.
While these HIPE-derived foams function very well in appropriately designed absorbent articles, the opportunity for further improvements associated with further advances in absorbent product designs is presented. In an example design, the entire storage element is to be located at a position distant from and relatively higher than the crotch region (referenced to a standing wearer). In diapers, such designs minimize the amount of fluid stored in the sensitive crotch region of the wearer. This not only preserves the original comfortable shape of the diaper, but also allows the crotch region to be thinner than it would otherwise be required. Perhaps most importantly, this design can keep the crotch region drier for better skin health because the urine is substantially removed from that location by the storage core. This design requires a component within the storage core that is able to drain the acquisition layer and then wick the absorbed fluid by capillary action to some significant height above the insult zone. Exemplary distribution materials are described in more detail in co-pending U.S. patent application Ser. No. 09/042,418, filed Mar. 13, 1998 by DesMarais et al. entitled "ABSORBENT MATERIALS FOR DISTRIBUTING AQUEOUS LIQUIDS" (P&G Case 7051). The storage element is then to be concentrated relatively nearer the wearer's waist than the crotch region. Thus, the product design comprises a relatively open acquisition member able to accept fluid gushes, a distribution layer with sufficient capillary pressure to remove the fluid substantially from the acquisition component and then wick the fluid to the height required before the next fluid insult is likely to have occurred, and a storage layer able to remove the fluid from the distribution layer. (The components are understood generally to be "layers" but can in principle comprise any shape consistent with the functioning of the product.) The absolute height required in this example is variable depending both on the size of the product and its core and the orientation of the wearer relative to the ground. In general, the height can range from about 12 cm for infants (who are seldom in an upright position) to about 15 to 20 cm for older toddlers and up to about 25 cm for adults. The storage element must be able to at least pull fluid against the hydrostatic head pressure created by these heights. Additionally, however, the storage element must overcome the desorption pressure of the acquisition or distribution layer against which it is positioned. See co-pending U.S. patent application Ser. No. 09/042,418, filed Mar. 13, 1998 by DesMarais et al. entitled "ABSORBENT MATERIALS FOR DISTRIBUTING AQUEOUS LIQUIDS" (P&G Case 7051), which is directed to improved fluid distribution materials. This desorption pressure for any material capable of wicking fluid 20 cm will be greater than 20 cm. The desorption pressure can be twice or more the absorption pressure.
While specific examples will vary, experience has shown that the storage element must be generally able to acquire aqueous fluids of nominal surface tensions against a total pressure (desorption plus gravitational) of at least about 40 cm, preferably at least about 50 cm, more preferably at least about 60 cm, and most preferably at least about 70 cm. In those absorbent articles wherein the crotch region comprises only fluid acquisition and/or distribution materials (i.e., no true storage material), the degree of dryness in the crotch area ultimately achieved is a function of the desorption pressure of the acquisition and/or distribution components, the height of the storage element relative to the acquisition component, and the absorption pressure and capacity of the storage element. Storage elements capable of delivering the capillary pressure necessary to generate a high level of dryness may be generally described as "high suction" elements.
The overall capacity of the storage element is also quite important. While many materials such as fibrous webs may be densified so as to acquire fluids against a total pressure of about 40 to 70 cm, the capacity or void volume of such components is poor, typically less than about 2-3 g/g at 40 cm. Densification also decreases the capacity at 0 cm. Further, such webs tend to collapse under pressure (hydrostatic and mechanical) due to poor mechanical strength, further reducing their effective capacities. Even the absorbent foams described in the art for use as storage components tend to collapse when subjected to pressures equivalent to more than about 30-40 cm of hydrostatic pressure. (Hydrostatic pressure is equivalent to mechanical pressure wherein 1 psi (7 kPa) mechanical pressure is equivalent to about 70 cm of hydrostatic pressure.) This collapse again substantially reduces (usually by a factor of between about 5 and 8) the useful capacity of these foams. While this reduced capacity can in principle be overcome by use of more absorbent material, this is generally impracticable due to cost and thinness considerations.
A third important parameter for a storage material is the ability to stay thin prior to imbibing aqueous fluids, expanding rapidly upon exposure to the fluid. This feature is described in more detail in U.S. Pat. No. 5,387,207, supra. This affords a product which is relatively thin until it becomes saturated with fluid at the end of its wearing cycle. This "thin-until-wet" property is contingent upon the balancing of capillary pressures developed within the foam and foam strength, as described in U.S. Pat. No. 5,387,207, supra.
Another important characteristic of the storage element is the ability to wick fluid within itself. Wherein the overlap between the acquisition or distribution component and the storage element is only partial, the storage component must itself be able to wick fluid throughout itself to be efficient.
Finally, it is desirable that the storage element be sufficiently tough to survive during use and manufacture, sufficiently flexible to be comfortable, and amenable to manufacture using commercially viable procedures for large scale production.
Accordingly, it would be desirable to be able to make an open-celled absorbent polymeric foam material that: (1) shows a relatively high capacity for aqueous fluid at relatively high heights; (2) can be relatively thin and lightweight during normal storage and use until wetted with these body fluids; (3) can wick fluids within itself; (4) has sufficient resiliency, toughness and strength under compressive load to absorb and retain these body fluids; and (5) can be manufactured economically without sacrificing these desired absorbency and mechanical properties to an unacceptable degree.